GH-3994 precompute hashCode for MemIRI

core/sail/memory/src/main/java/org/eclipse/rdf4j/sail/memory/model/MemIRI.java

@@ -42,7 +42,7 @@ public class MemIRI extends MemResource implements IRI {
 	/**
 	 * The MemURI's hash code, 0 if not yet initialized.
 	 */
-	private volatile int hashCode = 0;
+	private final int hashCode;
 
 	/**
 	 * The list of statements for which this MemURI is the predicate.
@@ -69,6 +69,7 @@ public MemIRI(Object creator, String namespace, String localName) {
 		this.creator = creator;
 		this.namespace = namespace;
 		this.localName = localName;
+		this.hashCode = (namespace + localName).hashCode();
 	}
 
 	/*---------*
@@ -147,10 +148,6 @@ public boolean equals(Object o) {
 
 	@Override
 	public int hashCode() {
-		if (hashCode == 0) {
-			hashCode = stringValue().hashCode();
-		}
-
 		return hashCode;
 	}